1. Field of the Invention
The present invention relates to a plating method and a plating apparatus for plating a surface of a substrate, and more particularly to a plating method and a plating apparatus for forming a plating film in fine interconnect trenches, holes, or resist openings provided on a surface of a substrate, such as a semiconductor wafer, or for forming bumps (extruding electrodes), to be electrically connected to electrodes of a package, on a surface of a semiconductor wafer. A plating method and a plating apparatus according to the present invention can also be used for filling via holes with metal in manufacturing of an interposer or spacer, having a large number of via plugs penetrating through it, which is to be used in three-dimensional packaging of semiconductor chips.
2. Description of the Related Art
It is a common practice in TAB (tape automated bonding) or flip chip to form protruding connection electrodes (bumps) of gold, copper, solder or nickel, or of multiple layers of such metals at predetermined portions (electrodes) of a surface of a semiconductor chip, having interconnects formed therein, so that the semiconductor chip can be electrically coupled via the bumps to electrodes of a package or TAB electrodes. There are various methods usable for formation of bumps, such as electroplating, vapor deposition, printing, and ball bumping. In particular, electroplating, which can form fine bumps and can be performed in a relatively stable manner, is widely used as the number of I/O in a semiconductor chip increases and the electrode pitch becomes smaller.
The electroplating method can be classified roughly into a jet method or cup method in which a substrate, such as a semiconductor wafer, is held in a horizontal position with a surface to be plated facing downward, and a plating solution is jetted upward onto the surface to be plated, and a dip method in which a substrate is held in a vertical position in a plating bath and a plating solution is injected upward into the plating bath while the plating solution overflows the plating bath during plating. Electroplating using the dip method has advantages of a small footprint and good release of bubbles which could adversely affect a quality of plating, and is therefore considered suitable for bump plating in which plating is performed on relatively large-sized holes and which requires a considerably long plating time.
A conventional electroplating apparatus using the dip method has a substrate holder for detachably holding a substrate, such as a semiconductor wafer, with its front surface (to-be-plated surface) exposed and a peripheral portion of the surface sealed. The electroplating apparatus is configured to perform plating of the surface of the substrate by immersing the substrate, together with the substrate holder, in a plating solution. The electroplating apparatus has the advantage of good release of bubbles.
Since the substrate holder is kept immersed in the plating solution during plating, the peripheral portion of the substrate, held by the substrate holder, must be securely sealed so that the plating solution will not intrude into a back surface (a surface opposite to the surface to be plated) and a peripheral area of the substrate where electrical contacts are in contact with the substrate. For example, the substrate holder has a pair of support members (holding members) designed to detachably hold the substrate therebetween and is provided with sealing members in one of the support members. One of the sealing members is brought into pressure contact with the other support member to seal a gap between the support members. The other sealing member is brought into pressure contact with the peripheral portion of the substrate, which is placed and held on the other support member, to seal a gap between the substrate and the support member.
In order to prevent leakage of a processing liquid (e.g., a plating solution) in such substrate holder, there has been proposed several approaches including: optimization of a shape and a fixing manner of the sealing member; regular cleaning of the sealing member (e.g., after every processing); regular replacement of the sealing member; improvement of accuracy of pre-processing (formation of a seed layer or a resist film) of a substrate; minimization of a positional error on setting of the substrate in the substrate holder; and regular readjustment of the sealing member.
However, it is fairly difficult to achieve completely sealed state because of deterioration of the sealing member or other causes. Especially when performing plating of a substrate surface to fill fine recesses, such as trenches or via holes, with a plating film, a plating solution having good permeability is typically used so that the plating solution can easily and securely enter the fine recesses. Use of such a plating solution makes it more difficult to establish the completely sealed state. It is also generally difficult to detect leakage of the plating solution into the substrate holder. Once the leakage of the plating solution occurs, the plating solution will intrude into the substrate holder and adhere to the peripheral portion and the back surface of the substrate. The plating solution attached to the substrate may be transferred to a substrate transport device and may contaminate the plating apparatus in its entirety. In addition, the leaked plating solution may corrode electrical contacts, resulting in poor feeding of electric current.
The applicant has proposed a substrate holder having at least one pair of electrical conductors, which are to be short-circuited by a leaked plating solution, in order to detect (through passage of electric current between the electrical conductors) leakage of a plating solution upon actual plating of a substrate held by the substrate holder, carried out by immersing the substrate, together with the substrate holder, in the plating solution (see Japanese Patent Laid-Open Publication No. 2004-52059). The applicant has also proposed a substrate holder which is configured to supply a pressurized gas into a space formed between a substrate, held by the substrate holder, and the substrate holder and surrounded by a sealing member, and to detect leakage of the gas from the sealing member by detecting a decrease in the pressure of the gas (see Japanese Patent Laid-Open Publication No. 2003-277995).
A pre-plating test of leakage of a plating solution has been proposed. In particular, after sealing a peripheral portion of a substrate with a sealing member, the possibility of leakage of a plating solution into a space defined by the sealing member is checked e.g. by depressurizing or pressurizing a hermetic space formed by the sealing member (see Japanese Patent Laid-Open Publication No. 2002-531702).
A pre-plating test of the integrity of sealing has been proposed. In particular, when a substrate holder holds a substrate while sealing a peripheral portion of the substrate with a sealing member, an internal space in the substrate holder, defined by the substrate, is depressurized e.g. by applying a small vacuum of about −0.05 atm to a recess in which the substrate is housed. The internal space is then closed off. If a change in the vacuum is within a predetermined level, e.g. 10%, within a certain time period, e.g. 5 seconds, then the integrity of sealing is determined to be verified (the leakage test is passed) (see Japanese Patent Laid-Open Publication No. 2007-509241).
It is generally difficult to determine the sealed state of the sealing member merely by measuring the pressure in the internal space in the substrate holder after evacuating or pressurizing the internal space. Moreover, when a trace amount of plating solution has leaked, a volume ratio of a volume of the internal space to the amount of the leaked plating solution is considerably high and, inversely, a change in the pressure in the internal space is considerably small. For example, when the volume of the internal space is 500 cc and the amount of leaked plating solution is 0.05 cc, the change in the pressure in the internal space is 1/10000. Therefore, even a high-precision pressure sensor could fail to detect the sealed state. In particular, the leakage of the plating solution with even a trace amount is required to be securely detected either before or after plating in order to continuously and stably operate the plating apparatus.
The leakage of the plating solution in the substrate holder can cause corrosion of the electrical contacts of the substrate holder, an increase in the electrical contact resistance, and the like. Therefore, in a case of conducting a post-plating leakage test of the plating solution, a maintenance work, including cleaning of the substrate holder and replacement of parts, may be required depending on the degree of the leakage of the plating solution.